The continuing development in the field of mobile radio technology has led to particularly compact and light mobile radiotelephone, so-called "mobile phones", and cordless telephones that are operated by batteries with high energy density. In the interests of a high degree of mobility and a continuous use, batteries of this kind must have a high capacitance and must be rechargeable in an extremely short time by means of public network energy or the on-board voltage of a vehicle. Since the batteries should also already have sufficient energy for the transmitting operation after a short charge time, an extremely light and compact charging unit is necessary, which for so-called quick charges supplies a relatively large amount of charging energy into the mobile device. Due to the compactness of mobile radiotelephones, the charging energy must be as lossfree as possible and must be conveyed to the battery with a minimum of control means in the phone device, i.e. the charging unit must at least have a current limitation. In the interests of reliability and comfort, the mobile device is advantageously intended to be coupled to the charging unit without electrical contacts. For a continuous reachability by incoming calls, the device must additionally be ready to receive even during charging. The charging unit is not permitted to influence the function of the connected mobile radio device, as is possible, for example, due to electromagnetically radiated interference from the harmonic waves of the switching pulses and can only be prevented in conventional switching converters by means of costly filters and shields.
The requirements mentioned are fulfilled particularly well by charging units which contain a DC converter wired with L-C-resonance circuits, since these converters feature low interference emission in the high frequency range and a low power loss. In addition, in comparison to other switching converters, this special embodiment of switching converters, a so called resonance converter, does not cause any rapid current and voltage changes and can therefore be operated at higher switching frequencies. This makes it possible, that a transformer can be used for the resonance converter which is very low in volume and weight.
In general, in resonance converters, at least the primary part of the transformer, together with a circuit capacitance, constitutes an oscillating circuit. In case when the resonance converter uses a push-pull oscillator, two switches periodically connect this oscillating circuit, to input DC voltage so that a recharging occurs periodically between the circuit capacitance and primary part.
In resonance converters, however, there is a known problem in that the resonance frequency depends not only on the inductance of the primary winding of the transformer and the circuit capacitance, but also on the secondary load. With increasing secondary power consumption, the resonance frequency increases in a resonance converters. Consequently, the resonance frequency coincides with the supplied control frequency only in a narrow load range. If the resonance converter functions outside this range, either the resonance current breaks off prematurely or the switches are heavily loaded as a result of the incorrect trigger timing. Also heavy energy losses can occur when switching over if both of the control electrodes of the switches are temporarily conductive, for example as a result of parasitic storage capacitances. In order to prevent the disadvantages mentioned, additional control means and/or protective measures such as protective diodes are required. The latter lead to additional energy losses and increase the interference emission in the high frequency range.
Various embodiments are known for the prevention of this disadvantage. For example, the reference EP 0 293 874 B 1 has disclosed a process and a circuit arrangement for status control for an oscillating circuit in a resonance converter. A costly control circuit monitors the current and/or voltage behavior in the resonance circuit by means of an inductive current-voltage converter disposed in series in relation to the primary winding and generates a triggering frequency for the switches, which is continuously modulated to the changing natural frequency. The resonance condition is maintained over a large load range.
The references EP 0 589 911 B1 and EP 0 589 912 B1 have disclosed switch regulators which contain a resonance converter with a push-pull oscillator, which is powered by a pre-regulator that is scanned with a pulse-width modulated signal. The pre-regulator decreases an intensely fluctuating input voltage and contains two individual inductances for uncoupling the feed of the input currents into the push-pull branches. With separate circuit capacitances, the two primary windings of a push-pull transmitter that are galvanically separated from each other each constitute a secondary-side resonance circuit. The circuit capacitances respectively apply regulated operating currents to the inputs for the current feed and have DC potential. Two switches alternatingly switch the primary windings of the push-pull branches in relation to ground. Between the switch changes, both of the switches are without current during a so-called gap time. In accordance with the description, the gap time should permit an oscillation of the push-pull converter including parasitic winding capacitances or capacitances of rectifiers that are not explained in detail. A control device clearly excites the switches independent of the load. The voltages that are present at the circuit capacitances and are added in a summing network, and the input current of the pre-regulator are used as control criteria for the pulse-width modulation. The input current of the pre-regulator is detected by an inductive current converter.
Inductive charging devices for mobile radiotelephones have also been disclosed. For example, the reference GB 2291 291A has disclosed a non-contacting battery charging device for supplying electric power from a charger without direct contact to an accumulator battery in a radio telephone. A charging unit comprises a primary coil, an oscillator for supplying AC power to the coil and an oscillator control section connected with means for turning on and off the power supplied to the oscillator. The radiotelephone contains circuits for generating a halt signal that halts the supply of AC power to the oscillator automatically via an optical link by an incoming call or by operating keys of the telephone keyboard. That eliminates the attractive force caused by electromagnetic induction between the radiotelephone and the charger to enable easy removal of the device from the charger. In an especially embodiment of the known solution a contact switch arranged in a depression of the charger turns the charger on when the radiotelephone is placed within the charger.
Charging devices of the prior art transmit only a low electrical charging power to the radiotelephone, which is insufficient for quick charges, although the devices contain voluminous primary and secondary windings. In addition, these known charging devices have a high-energy consumption and an intense magnetic interference emission in relation to the power transmitted.
The operating mode of a charging device with high power transmission creates a great problem for operational reliability, when the device does not switch into a low-power standby mode automatically.
Also, in the no-load state, i.e. when a charging control in the mobile device has ended the charging process, or when the mobile electrical device has been removed from the charging unit, the charging unit requires a considerable amount of power so that in continuous operation, additional measures are necessary for a continuously reliable bleeding off of power. Furthermore, foreign bodies that are electrically and/or magnetically conductive and get into the alternating field by chance, for example coins, metallic office accessories and the like, can absorb a lot of energy from the alternating field, heat up intensely due to inductively generated short circuit currents and eddy currents, and represent a danger for the surrounding area. Even a mobile telephone or another electronic device that is not equipped for inductive energy transmission can inadvertently get into the region of the alternating field and possibly even get damaged due to a continuous inductive heating. On the other hand, by shifting the resonance condition, the foreign bodies can considerably increase the power consumption of the charging device so that it is destroyed when there is insufficient heat dissipation.
Known resonance converters have expensive control circuits with additional inductive components in order to precisely control the switches in the load range from no-load to full-load. Furthermore, the known resonance converters are only insufficiently equipped with control means that permit a primary-side detection of and reaction to changes in the load by means of the secondary, e.g. the removal of the mobile electrical device after the charging of the batteries is finished.